PHOTORESPONSES OF THE PURPLE NONSULFUR BACTERIA RHODOSPIRILLUM-CENTENUM AND RHODOBACTER-SPHAEROIDES

We have measured the photoresponse of two purple nonsulfur bacteria, R hodobacter sphaeroides and Rhodospirillum centenum, under defined cond itions in a light beam propagating at 90 degrees to the optical axis o f the microscope. This beam presented cells with a steep gradient of i ntensity perpendicular to the direction of propagation and a shallow g radient in the direction of light propagation. R. centenum, a species that reverses to change direction, accumulated in the light beam, as e xpected for a ''scotophobic'' response, while R. sphaeroides, which st ops rather than reverses, accumulated outside the light beam. We also compared the behavior of liquid-grown R. centenum, which swims by usin g a single polar flagellum, to that of surface-grown R. centenum, whic h swarms over agar by using many lateral flagella and has been shown t o move as colonies toward specific wavelengths of light. When suspende d in liquid medium, both liquid-and surface-grown X. centenum showed s imilar responses to the light gradient. In all cases, free-swimming ce lls responded to the steep gradient of intensity but not to the shallo w gradient, indicating they cannot sense the direction of light propag ation but only its intensity. In a control experiment, the known photo tactic alga Chlamydamonas reinhardtii was shown to swim in the directi on of light propagation.